Expulsion fuse



J. SANDIN EXPULSION FUSE Jan. 23, 1934.

Filed Sept. 26, 1929 222E2522..$2222,22E5.2Eiszfi? 52?? s 2 .I

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ENVENTOR fmma 54174522.

1 ATT'ORNEY Patented Jan. 23, 193 4 1,944,762 nxPULsIoN FUSE Jerome Sandin, Forest Hills, Pa., asslgnor to Westinghouse Electric 8; Manufacturing Company, a corporation of l 'ennsylvania Application September 26, 1929 Serial No. 395,266

2 Claims. (01. 200-121) Y My invention relates to fuses and particularly to fuses of the expulsion type wherein a fusible element in the form of wire or a strip is enclosed within an insulating tube that is closed at one end by a terminal member for the fuse and which is commonly used to interrupt hightension circuits, the point of rupture of the fuse being within the tube surrounding the fuse element and the gases incident to rupture of the m fuse being discharged from the open end of the tube;

An object of my invention is to provide a high tension expulsion fuse that is adapted for lowcurrent protection, that is, an expulsion fuse in which fus'ble elements having relatively lowcurrent ratings may be used and which safely interrupt overload or low currents after they have endured a predetermined time.

A further object of my invention is to provide an expulsion fuse wherein the outer enclosure of one size is adapted for use with fusible refills having a relatively largejnumber of d fferent capacities. thus reducing the number of different sizes of expulsion fuse enclosures required to be retained in stock.

A further object of my invention is to provide an expulsion fuse wherein the fuse is adapted for overload protect on by means of an insulating inner tube which surrounds the fusible element and which is designed to remain intact and withstand interior pressures generated by rupture of the fuse element at a point within the tube under low current or overload conditions and which fractures under severe short-c rcuit conditions to facilitate interruption of the circuit under such conditions.

A further object of my invention is to provide a refill for expulsion fuses wherein the inner tube surroundng the fusible element con- 40 stitutes a part of the refill and serves among other uses, to protect the fuse element, and wherein the refill is made of such fibrous material that it is self -cleaning under the influence of the metalladen gases of high temperature arcs and which,

0 therefore, will not receive and retain a metallic conductive deposite such as usually condenses on the interior surface of fuse tubes and renders the tube ineffective for interrupting the circuit.

5 A further object of my invention is to provide a refill for expulsion fuses having the above-noted characteristics wherein the insulating tube is made of fish paper or other material made of cotton fiber which generates an un-ionized gas 55 when heated or burned and which will not permit of deposit of metallic vapors in the are incident to rupture of the fusible element.

A further object of my invention is to provide an expulsion fuse wherein the length and di-- ameter of the bore of the tube are proportioned with respect to the rating and size of the fusible element that a predetermined pressure of such magnitude is maintained within the inner tube during the process of interruption of the circuit, that it will successfully cause the arc to be extinguished in a minimum of time and with a minimum generation of metallic vapors.

It has been found that the proportions of the length and internal diameter of the insulated tube immediately surrounding the fusible element is a determining factor in the successful operation of an expulsionfuse. It has also been found that, where the fusible element is confined within a tube having a-relatively small bore required for successful functioning of the fuse under low-current or overload conditions, such tubes are not as successful in opening the circuit under short-circuit conditions as a tube having a. considerably larger bore, such as the outer-enclosing tube of the usual expulsion fuse.

It has further been found very difiicult to construct an insulating tube having a relatively small bore and of sufficient strength to withstand the violent explosion usually incident to rupture of the fuse under short circuit conditions. When the walls are made of sufficient strength to prevent actual bursting of the fuse tube, the internal pressures are so extremely high that they break down the fiber of the interior walls of the tube in such manner as to render the tube worthless for a second operation.

From a practical standpoint, aside from the fact that fire is thrown a considerable distance from the fuse tube of small bore under shortcircuit conditions, such tubes are not commercially practicable because of the relatively large expense of the heavy tube compared with that of a small inner tube of relatively light and inexpensive construction.

In United States Patent No. 1,684,518, filed October 25, 1921, there is disclosed an expulsion fuse having an inner tube that is adapted to be discharged from the open end of the outer tube under overload or short-circuit conditions, the purpose being to provide a means for increasing the length of the arc path. The inner tube was provided primarily for that purpose and the importance of providing a tube having a relatively small bore in which suiiicient pressure is gener- 110 ated to readily extinguish the arc, was not appreciated.

While expulsion fuses of the type disclosed in the above-noted patent operate successfully, the operation of an expulsion fuse made in accordance with my present invention interrupts the circuit under both overload and short-circuit conditions in a superior manner and with less violence of discharge than the expulsion fuse disclosed in the patent.

In the expulsion fuses at present used, it has been the practice in some instances to surround the fuse element or strip with a thin envelopemade of asbestos fiber or the like for the purpose of preventing burning of the'i'nner wall of the expulsion tube surrounding the'fuse element. This shield ruptures under overload? and shortcircuit conditions and so far as I haveibeen able to determine is of no particular assistance in interrupting the circuit.

It is an object of my invention, therefore, to produce an expulsion fuse that is free from the objectionable features of fusesbf the prior art referred to above and to provide a fuse refill that is inexpensive and which may be readily assembled in the expulsion fuse casing and which serves to interrupt an electric circuitunder overload and short-circuit conditions. i

These and other objects that will be made apparent throughout the further description of my invention are attained by means of the apparatus hereinafter described and illustrated in the accompanying drawing wherein: 1

Figure 1 is a longitudinal section through an expulsion fuse embodying features of my invention;

Fig. 2 is a longitudinal section through the refill fuse element shown in Fig. 1; i

Fig. 3 is a bottom end view of the fuse refill shown in Fig. 2; and

Fig. 4 is a longitudinal section through a modified form of fuse refill. a

Referring to the drawing, the expulsion fuse comprises an insulating tube or casing 5 that is threaded at one end for receiving a. threaded terminal bushing 6 having a bore '1 therethrough for receiving a fuse refill 8, the construction of which will hereinafter be described. The outer surface of the terminal 6 is rectangular in crosssection and is adaptedto fit within the usual resilient fuse clip carried by the door of the housing in which the expulsion fuse is usually mounted. The end of the terminal 6 is threaded for receiving a closure cap 9 which serves to close the upper end of the expulsion fuse tube hand to clamp the terminal 11 of the fuse refill 8 in conductive engagement with the terminal bushing 6.

The fuse refill comprises a terminal member 11 having a reduced portion 12 that is threaded for receiving the threaded end of an insulating auxiliary or inner tube 13 which serves to substantially surround the fuse element 14 thatis attached at its inner end to the reduced portion 12 of the fuse terminal by means of solder 14'.

The fuse element comprises a strand of lead or zinc wire to which is soldered at 16 a. second strand 17, the strands being twisted together as indicated in Figs. 1 and 2, thus rendering the fuse strip relatively flexible and providing a portion at the inner end of the fuse that is of less cross-sectional area than the major portion of the fuse element. The purpose of providing a portion of the fuse with a reduced cross-section at its inner end is to insure rupture 01' the fuse element adiacent the inner closed end of the auxiliary tube 13.

The outer ends or strands are soldered to a flat copper fuse terminal strip 18 in a manner to be hereinafter described, the terminal 18 being made of flexible material and bendable into engagement with a terminal sleeve 19 secured exteriorly of the tube 5 and provided with a threaded clamping nut 21 mounted upon a screw 22 that is attached to the terminal sleeve.

The outer end of the terminal strip 18 is provided with a notch 23 through which the screw 22 projects for preventing lateral displacement of the fuse strip during tightening operation of the threaded nut 21 which serves to clamp the terminal 18 into engagement with the terminal sleeve 19.

During the process of manufacture of the fuse refill, the fuse element 14 is first soldered to the terminal 11, the insulating tube 13 is then screwed upon the terminal. The open end of the fuse tube 13 is flattened slightly for receiving the flat terminal 18, the attached end of which is extended into the tube, as indicated in Fig. 2. The terminal 18 is then bent to the dotted line position shown in Fig. 2 in which position the twisted strands of the fuse element are soldered to the strip at points exterior of the tube. The terminal strip 18 is then straightened into the position shown in full lines in Fig. '2. The purpose of soldering the fuse strands to the strip while in the bent position is to insure sufficient slack in the fuse element within the tube to prevent imposing strains upon it when the terminal 18 is bent into operative clamping position with respect to the terminal sleeve 19, as indicated in Fig. 1.

The insulating tube 5 may be made of insulating material such as fibre and is of relatively large diameter compared with the internaldiameter of the auxiliary fuse tube. The walls of the tube 5 are of sufficient strength to withstand the pressures generated by the explosion of the fuse element incident to a severe short circuit, since the large bore of the tube prevents destructive pressures being generated within the tube wall. However, suflicient pressure is maintained to extinguish the arc almost instantaneously.

It has been found, however, that where the fuse element is ruptured under low-current or over-load conditions in a tube of large bore, the generation of pressure is insufficient to promptly extinguish the arc with the result that the fuse strip slowly burns and fuses, giving off metallic metal-laden vapors which tend to sustain the arc through a destructive length of time. Under such conditions, the outer tube may be badly burned and coated with metallic deposits.

In order to overcome this difficulty and to render a fuse casing of one, size suitable for interrupting circuits under overload conditions with 135 fuse elements of dlfferent rated capacities, I have provided a fuse refill having an auxiliary or inner insulating tube that substantially surrounds the fuse element throughout the length of the outer tube and in which the fuse is ruptured 140 adjacent the closed end thereof. It has been found that by proportioning the internal diameter and the length of the auxiliary tube with respect to the size of the fuse wire or the rated capacity thereof, overload circuits may be successfully and substantially instantaneously interrupted withoutdestroying or materially damaging the outer casing of the fuse.

It has also been found that micarta or similar w insulating materials are unsuitable for use as an auxiliary tube for the reason that they become coated with metallic deposits which condense from the arc gases and thus become conductive and, therefore, useless because they provide a conductive path for the current and sustain the are through a destructive length of time.

I have-found that insulating materials made of cotton fiber, such as fish paper, may be used successfully for the inner tube because the fish paper burns or volatilizes when the arc comes in contact with it and gives off an un-ionized gas that prevents the deposit of metal on the interior surface of the tube.

Three factors, therefore, enter into the successful interruption of a circuit under overload or low- "current conditions, namely, a tube having its internal diameter and length proportioned to insure the proper pressure within the tube to assist extinction of the arc; the combustion within the tube of a cotton fiber insulating material that gives off un-ionized gas which facilitates extinction of the arc and; the provision of a tube of sufficient strength to withstand the pressures generated under overload or low-current conditions and which prevents the deposit of metal particles on the inner walls during the operation of-extinction of the arc. These factors materially contribute to the extinction of the arc and when all three are brought into combination produce a device that is admirably adapted for successfully and satisfactorily interrupting the circuit under low-current or overload conditions in a highpotential circuit.

It has been pointed out that where attempts have been made to open a circuit under severe short-circuit conditions by a fuse confined in a tube suitable for opening the circuit successfully under overload conditions, the-pressures generated within the tubeof small bore are so extremely high that the inner lining of the tube is destroyed, rendering it useless for further service, and that the explosion is so violent that it becomes destructive to theapparatus enclosing the fuse and throws fire dangerous distances from the open end of the expulsion fuse tube.

It is, therefore, better to provide a larger space within the expulsion fuse tube'in order to prevent generation of extremely high pressures and destruction of the tube under short circuit conditions. For this reason, the walls of the auxiliary tube are of such thickness and strength that ing pressures are'attained by the use of an inner tube having an internal diameter of 5%" and For fuse elements having ratings of 15 to 60 amperes, the internal diameter of the tube is substantially 1%". As an example of the importance of the diameter of the tube and the pressure generated within the tube, it has been found that for the extremely low currents rang ing from 1 to 10 amperes, a tube of internal diameter does not satisfactorily operate.

For fuses of higher rated capacities, larger internal diameters and lengths of tube are retube of one size may be used in combination with fuses of different ratings within a certain range limit, a predetermined pressure'within certain range limits must be maintained within the inner tube in order to produce successful extinction of the are under low-current or overload conditions.

In Fig. 4 I have shown a modified form of refill having a fuse element adapted for relatively large currents in the order of to 150 amperes. The fuse refill comprises a terminal 24 to which is riveted a composite fuse strip 25 formed of a relatively thick strip of zinc 26 and a thin strip of copper 27. The insulating tube 13 is loosely mounted on the fuse strip and is retained in position in the tube by bending the projecting end of the fuse strip.

A composite fuse strip is used for the reason that if a strip of zinc were used having sufficient .current carrying capacity, its thickness would be so great that it would not bendreadily and would be hard to manipulate during the process of inserting it in the enclosing casing. In operation, the zinc strip fuses first and subsequently the increased current in the copper strip causes it to rupture almost instantly after the zinc strip has ruptured. Under overload and short-circuit conditions, the fuse strip otherwise functions the same as the fuse elements shown in Fig. 1. In order to insure that the fuse ruptures at a point adjacent the terminal or inner end of the tube 13, the zinc and copper strips are provided witha perforation 28 which reduces the cross-section of the fuse strip at that point.

It is apparent that I have provided an expulsion fuse for high potential circuits that is capable of successfully opening the circuit under both overload or low-current conditions and short-circuit conditions and have produced an expulsion fuse that may be used throughout a large number of current ratings of relatively small and large capacities and have produced an expulsion fuse that requires a minimum number of outer casing sizes in order to accommodate a comparatively large range of current capacities.

The refill is simple and relatively inexpensive in construction and may be readily replaced.

While I have illustrated but two embodiments of my invention, it will be apparent to those skilled in the art that various changes, modifications, substitutions, additions, and omissions may be made in the apparatus illustrated without departing from the spirit and scope of my invention as set forth in the appended claims.

I claim as my invention:

l. The combinat'on with an expulsion fuse having an open end insulating outer tube and a fuse -.;i"element therein adapted to rupture under a prea length of 6%", where fuse elements are usedfi having a rated capacity of from 1 to 10 amperes.

ing an insulating outer tube and a fuse element therein adapted to rupture under a predetermined current value, of an insulating inner tube within the main tube surrounding the fuse element and made of insulating fiber that gives off unionized gases when the tube is burned by the arc incident to rupture of the fuse element within the inner tube and which is of insufficient strength to withstand the pressures generated by an arc incident to rupture of the fuse under short circuit conditions, the said internal diameter and length of the inner tube being so proportioned with respect to the said predetermined current value at the time of rupture of the fuse element that the gases within the inner tube attain a predetermined arc extinguishing pressure, and said inner and outer tubes each having an end which is open after the arc is caused, to permit the gases to flow out said end.

3. The combination with an expulsion fuse having an open end insulating outer tube for confining arc gases incident to rupture of a fuse therein under short-circuit conditions and for discharging the gases out of the open end of the tube, and a fuse element within the tube adapted to rupture substantially instantaneously under short-circuit conditions of the circuit in which the fuse element is connected andtoruptureapredetermined time after the current value has increased under overload conditions to a predetermined amount above the rating of the fuse element, of an insulating inner tube within the main tube and surrounding the fuse element and made of an insulating fiber that gives off un-ionized gases when subjected tothe heat of the arc incident to rupture of the fuse element within the inner tube and which is of insufficient strength to withstand the pressure generated by an arc incident to rupture of the fuse under short circuit conditions.

4. A refill for an expulsion fuse comprising a fuse element and an insulating tube surrounding the element and made of combustible cotton fiber and having walls of such strength that it remains intact during rupture of the fuse element and interruption of the circuit under low current conditions, but is ruptured during interruption of the circuit under greatly excessive current conditions.

5. In a circuit interrupter, a pair of tubular members, one of which is positioned within the other, and a fusible element positioned within said inner tube, the said wall of said inner tube having such strength as to be ruptured when sub- Jected to the gas pressure caused by the blowing of the fusible element by a current greatly in excess of its rated value, but to be able to withstand the pressure caused by the blowing of the fusible element by a current slightly in excess of its rated value.

6. A refill for an expulsion fuse comprising a fuse element and an insulating tube surrounding said element, the side wall of said tube having such strength as to rupture due to the pressure of the gas formed when said fuse element is blown by a current greatly in excess of its rated value, and to be able to withstand the pressure of the gas formed when said fuse element is blown by a current slightly in excess of its rated value.

'I. In a circuit interrupter, an outer insulating tube having one end closed, an inner tube said outer tube and extending for the major portion of the length of said outer tube, said inner tube being of insulating material and having an innersurface which gives off a large quantity of V to rupture due to the gas when acted upon by the are, means within said inner tube for causing an arc therein under predetermined current conditions, and said inner tube being of sufficient strength to withstand the pressure of the gas formed under currents somewhat in excess of said predetermined current conditions and causing a blast of said gas out one end of said inner tube, and said inner tube being of insuiiicient strength to prevent bursting thereof under current conditions greatly in excess of said predetermined current conditions.

8. In a circuit interrupter, an outer tubeof insulating material, an electrical terminal on one end of said outer tube, said terminal having a bore therethrough for receiving a refill, a refill positioned in said outer tube, said refill comprising a removable unit including a fuse element and an inner tube surrounding said element, the side wall of said inner tube having such strength as to rupture due to the pressure of the gas formed when said fuse element is blown by a current greatly in excess of its rated value and to be able to withstand the pressure of the gas formed when said fuse element is blown-by a, current slightly in excess of its rated value, said removable refill unit being positionable in said outer tube through the bore of said terminal member, a cap member for closing the bore through said terminal member when the refillis in place, and said inner and outer tubes each having its end opposite said terminal open after the arc is caused to permit the flow of gas out of said tubes.

9. In a circuit interrupter, an outer tube of insulating material, a refill positioned in said outer tube, said refill including a removable unit including a fuse element and an inner tube surrounding said element, said inner tube being secured at one end to said fuse element, the side wall of said inner tube having such strength-as pressure of the gas formed when said fuse element is blown by a current greatly in excess of its rated value and to be able to withstand the pressure of the gas formed when said fuse element is blown by a current slightly in excess of its rated value, and said inner and outer tubes each having its end opposite said terminal open after the arc is caused to permit the flow of gas out of said tubes.

10. In a circuit interrupter, an outer tube of insulating material, a fuse element and an inner tube surrounding the fuse element positioned in said outer tube, said inner tube being of a combustible fiber and the side wall of said inner tube having such strength as to rupture due to the pressure of the gas formed when said fuse element is blown by a current greatly in excess of its rated value and to be able to withstand the pressure of the gas formed when said fuse element is blown by a current slightly in excess of its rated value, and said inner and outer tubes each having one end open after the arc is caused to permit the flow of gas out of said tubes.

11. In a circuit interrupter, an outer tube of insulating material, a fuse element and an inner tube surrounding the fuse element positioned in said outer tube, said inner tube having an outer diameter considerably less than the inner diameter of said outer tube and the side wall of said inner tube having such strength as to rupture due to the pressure of the gas formed when said fuse element is blown by a current greatly in excess of its rated value and to be able to withstand the pressure of the gas formed when said fuse element is blown by a'current slightly in excess of its rated value, and said inner and outer tubes each having one end open after the are is caused to permit the flow of gas out of said. tubes.

12. In a, circuit interrupter, an outer tube of insulating material, an electrical terminal on one end of said outer tube, said terminal havingend having such strength as to rupture due to time pressure of the gas formed when seizi Erase element is blown by a. current in exeess its rated. value and to be able to t pressure of the gas formed when said fess element is blown by a current slightly in exeess oi rated value, said removable eefill unit l sitioneble in said outer tube through time last" ef said terminal member, a oego member elo the bore through said. termmel member the refill is in place, and said o 2 tubes each having its end ozone-site s open after the are is caused to gas out of said. tuloes.

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